Reconsider whether forcing resources to return arrays is correct

[Bilge]

Currently Porter believes resources should always want to return structured data as an array. However, there may be use-cases where structured data is either unavailable or undesirable. I am yet to encounter any compelling cases but am very interested to hear about any such cases.

If we open up the return type to be mixed, this would allow resources to return objects, which would solve #12. Allowing objects can be convenient for object-oriented applications, but if resources return objects as the de-facto standard, this could be inefficient for applications that just want to work with raw data. However, mixed would even permit resources to return different types depending on some configuration parameter.

Forcing the array return type is nice because it feeds into the transformers subsystem, giving transformers a consistent type to work with. However, I'm willing to forgo the entire transformers system in a future version, or change it to only be available when the return type is array, or change it to work with any return type, as necessary. Ultimately, the consequences for the transformers system are not important because Porter's primary responsibility is fetching data reliably, not transforming it.